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Semiconductor Saturable Absorber Mirrors

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Acronym: SESAM

Definition: saturable semiconductor absorber devices acting as nonlinear mirrors

German: sättigbare Halbleiterspiegel

Categories: photonic devices, light pulses

How to cite the article; suggest additional literature

A semiconductor saturable absorber mirror (SESAM) (or simply SAM = saturable absorber mirror) is a mirror structure with an incorporated saturable absorber, all made in semiconductor technology. Such devices are mostly used for the generation of ultrashort pulses by passive mode locking of various types of lasers.

Typical Structure of a SESAM

Typically, a SESAM contains a semiconductor Bragg mirror and (near the surface) a single quantum well absorber layer. The materials of the Bragg mirror have a larger bandgap energy, so that essentially no absorption occur in that region. Such SESAMs are sometimes also called saturable Bragg reflectors (SBRs). For obtaining a large modulation depth, as required e.g. for passive Q switching, a thicker absorber layer can be used. Also, a suitable passivation layer on the top surface can increase the device lifetime.

SESAM structure

Figure 1: Structure of a typical SESAM for operation around 1064 nm. On a GaAs substrate, a GaAs/AlGaAs Bragg mirror is grown. Within the top layers, there is an InGaAs quantum well absorber layer, which may be e.g. 10 nm thick.

The penetration of the optical field into a SESAM can be calculated with the same matrix method as applied to other types of dielectric mirrors. Of particular importance is the optical intensity in the region where the saturable material is placed. This influences the modulation depth and also the saturation fluence (see below). However, the design of the structure also influences the bandwidth and the chromatic dispersion.

refractive index profile and intensity distribution in a SESAM

Figure 2: Refractive index profile and optical intensity distribution within a SESAM with anti-resonant design, as is often used. The intensity distribution has a maximum at the absorber position (indicated by the vertical gray line). The diagram has been made with the software RP Coating.

There are also some more exotic types of semiconductor saturable absorbers, which can be based on, e.g., quantum dots embedded in glass [10, 12] or on carbon nanotubes [13].

Resonant and Nonresonant SESAM Designs

As there is a Fresnel reflection at the semiconductor–air interface, this together with the Bragg reflection leads to a cavity effect (resonance effect). In most cases, this cavity is designed to be antiresonant for the operation wavelength of the device (see also Figure 2). Such devices exhibit a relatively broad wavelength range with a more or less constant degree of saturable absorption and with small chromatic dispersion. Compared with devices with an anti-reflection coating, antiresonant designs have a lower field penetration into the absorber and thus a lower modulation depth in addition to a higher saturation fluence and higher damage threshold. (The latter, however, is no advantage, because a higher incident pulse fluence is needed to saturate such a device.)

In relatively rare cases, resonant designs are used. These have a higher modulation depth and lower saturation fluence, and a smaller range of operation wavelengths.

By varying the material composition and certain design parameters, the macroscopic parameters of a SESAM (in particular, the operation wavelength, the modulation depth, the saturation fluence, and the recovery time) can be tailored for operation in very different regimes.

Physical Mechanism of Saturable Absorption

excitation and relaxation of a SESAM

Figure 3: Excitation and relaxation of carriers in a semiconductor.

The saturable absorption is related to an interband transition: the energy of absorbed photons is transferred to electrons, which are brought from the valence band to the conduction band. There is first some fairly rapid thermalization relaxation within the conduction and valence band within e.g. 50–100 fs, and later (often on a time scale of tens or hundreds of picoseconds) the carriers recombine, often with the aid of crystal defects.

For low optical intensities, the degree of electronic excitation is small, and the absorption remains unsaturated. At high optical intensities, however, electrons can accumulate in the conduction band, so that initial states for the absorbing transition are depleted while final states are occupied (Pauli blocking). Therefore, the absorption is reduced. After saturation with a short pulse, the absorption recovers, first partially due to intraband thermal relaxation, and later completely via recombination.

reflectivity change in a SESAM, caused by a short pulse

Figure 4: Reflectivity change of a semiconductor saturable absorber, hit by a short pulse at t = 0. Part of the reflectivity change disappears very quickly after the pulse, whereas another part takes many picoseconds to recover. Such curves can be recorded with pump–probe measurements.

Important Properties of SESAMs

The most important characteristics of a SESAM as used e.g. for passive mode locking or Q switching are the following:

Additional details concern the lateral homogeneity, the group delay dispersion (see below), the optical damage threshold and the device lifetime, also the suitability for high-power operation (see below). The lifetime of a SESAM is often difficult to assess and depends strongly on the operating conditions. Furthermore, it can be important that a SESAM can tolerate a certain heat load. Thermal issues become important not only at high average power levels, but also for operation with very high pulse repetition rates.

Semiconductor Materials for SESAMs

By far the most common type of SESAM is used in lasers emitting in the 1-μm wavelength region. Here, the saturable absorber is an InGaAs quantum well (or sometimes multiple quantum wells), where the indium content is adjusted to achieve an appropriate value of the bandgap energy. The mirror structure is based on GaAs and AlAs, grown on a gallium arsenide wafer. The lattice mismatch of InGaAs on GaAs and AlAs causes significant compressive strain in the absorber layer. Particularly for high indium contents, this can cause the formation of defects. The effect of defects may even be helpful, as it reduces the recovery time and may thus allow for shorter pulses and better pulse stability in a mode-locked laser. The defect concentration is therefore often increased by low-temperature growth of the absorber layer. For too low growth temperature and/or a high indium content, however, nonsaturable losses can become too high. The recovery time may also be reduced by bombardment with fast ions after growth (ion implantation). Partial annealing of defects at some elevated temperature can help to find a better compromise between nonsaturable losses and recovery time.

For use at shorter wavelengths, e.g. for passive mode locking of titanium–sapphire lasers emitting around 800 nm, GaAs quantum wells can be used. The use of GaAs then has to be avoided in the mirror structure; it is common to use a Bragg mirror made of AlGaAs/AlAs. For very short pulse durations, the reflectivity bandwidth of a Bragg mirror is not sufficient; in such cases, special broadband SESAM designs containing a metallic mirror are sometimes used.

At longer wavelengths such as the bands around 1.3 or 1.5 μm, InGaAs quantum wells can still be used, but they then have a very high built-in strain. Therefore, GaInNAs (dilute nitride) absorbers have been developed, which allow for very low nonsaturable losses. It is also possible to use indium phosphide-based absorbers in devices grown on InP wafers. Various types of Bragg mirrors are used in the 1.5-μm region, partially depending on the type of absorber layer.

SESAMs for High-power Operation

There are passively mode-locked high-power lasers with average output powers of well above 100 W and intracavity average powers of well above 1 kW. A SESAM used in such a laser will typically absorb between 0.2% and 2% of the incident power, and this can give rise to substantial thermal effects. In particular, there can be substantial thermal lensing, which affects the mode properties of the laser resonator. Also, the temperature increase may lead to accelerated aging or even optical damage. For such reasons, it is desirable to optimize SESAMs for use in high-power lasers [27] e.g. in the following ways:

Using such methods, it should be possible to employ SESAMs even in passively mode-locked lasers with average output powers of multiple kilowatts.

Note that a substantial local temperature increases can occur even in low-power lasers (e.g. with 1 W of average output power) if the pulse repetition rate is very high (many gigahertz). In that situation, one has to use relatively strong focusing of the radiation in order to achieve sufficiently strong saturation of the absorption despite the small pulse energy. It can then be challenging to cope with the heat generation despite the quite moderate absorbed average power. High-power lasers with much lower pulse repetition rate (normally some tens of megahertz) allow the use of much larger beam areas, making it much easier to handle substantial absorbed powers.

Dispersive SESAMs

Although most SESAMs exhibit only moderate amounts of chromatic dispersion for reflected light, dispersion of any sign can be engineered into a SESAM via the multilayer structure [7, 11]. Such dispersive SESAMs may then serve the purpose of dispersion compensation in a laser resonator, in addition to the function of a passive mode locker. However, such methods have only relatively rarely been applied, mostly because the need to control dispersion introduces certain design conflicts. For example, the wanted dispersion may only appear in a limited optical bandwidth, and wavelength-dependent losses of the device may drive the laser to operate outside that bandwidth. Also, it is restricting to work with SESAMs which offer certain fixed combinations of saturable absorption and dispersion.

Applications of SESAMs

SESAMs are widely used for passive mode locking of lasers, particularly for solid-state bulk and fiber lasers. They work with a wide range of laser parameters and usually allow for reliable self-starting mode locking, if their device and operation parameters are correctly chosen. They can be used even at very high output power levels of tens of watts, provided that the overall laser design allows them to be operated in the appropriate regime. Another application is passive Q switching, e.g. of microchip lasers or fiber lasers.

A general condition for the successful use of SESAMs in lasers is the selection of a suitable SESAM design and the adjustment of a number of laser parameters, in particular the resonator mode size on the absorber. The use of a SESAM with inappropriate device and operation parameters often leads to problems in the form of various instabilities or SESAM damage.

SESAMs can also be used for certain methods of nonlinear filtering and signal processing, e.g. in the context of optical fiber communications.


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[26]C. J. Saraceno et al., “SESAMs for high-power oscillators: design guidelines and damage thresholds”, IEEE J. Sel. Top. Quantum Electron. 18 (1), 29-41 (2012)
[27]A. Diebold et al., “Optimized SESAMs for kilowatt-level ultrafast lasers”, Opt. Express 24 (10), 10512 (2016)
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(Suggest additional literature!)

See also: saturable absorbers, passive mode locking, self-starting mode locking, Q switching, pulse generation, Spotlight article 2010-03-22, Spotlight article 2010-07-27
and other articles in the categories photonic devices, light pulses

In the RP Photonics Buyer's Guide, 2 suppliers for SESAMs are listed.

Dr. R. Paschotta

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